By development of optical fibers for optical amplification, in which an activation substance for optical amplification, for example, a rare-earth element is doped to the core, amplification was to made possible without converting optical signals to electrical signals. Optical fibers for optical amplification used for optical amplifiers in a usual optical transmission system are used at a length from several meters to several hundreds of meters, wherein in comparison with optical fibers used as optical transmission line, their usage length is short. Therefore, in conventional transmission systems, it has been considered that influences which chromatic dispersion of optical fibers for optical amplification gives to signal distortion are disregarded. Therefore, no special attention was paid to the chromatic dispersion characteristics in optical fibers for optical amplification; profile design (design of refractive index profile) has been carried out, highly considering the improvement of the amplification characteristics.
Generally in order to improve the characteristics, it is considered that it is effective to increase the relative refractive index difference .DELTA. of a core of optical fibers for optical amplification with respect to cladding and make the core diameter small. This results from the fact that the confinement effect of light is increased by increasing the relative refractive index difference .DELTA. of a core with respect to cladding to cause the pump power density (pump power) in an optical fiber for optical amplification to be enhanced, and a favorable population inversion is formed all over the area to which an activation substance for optical amplification is doped, by concentratedly causing the activation substance to exist at a higher place of pump power density while making the core diameter small, thereby improving the amplification characteristics (amplification efficiency).
As an example of a profile of optical fibers for optical amplification, which is designed in view of improvement of the amplification characteristics there is available a stepped type (which may be called matched type). Generally, optical fibers for optical amplification are formed of a silica (SiO.sub.2) glass, (pure silica) based core, to which germanium is highly doped, and silica glass based cladding to which fluorine is highly doped. The relative refractive index difference .DELTA. of the core with respect to the clad is 2.0% or so. The core diameter of the abovementioned optical fibers for optical amplification is about 2.7 .mu.m and the outer diameter of the fibers is 125 .mu.m. Furthermore erbium which is one of the activation substances for optical amplification and aluminum which averages the wavelength dependency of amplification gain, are doped to the core.